One method of heating greenhouses is the use of overhead unit heaters. These heaters operate off hot water, natural gas, propane or oil and function by blowing hot air from the heaters into the greenhouse environment. The heated air is then mixed throughout the greenhouse by use of horizontal air flow (HAF) fans or fan jets with poly convection tubes (which are not as common). These heaters are a highly efficient heating system when considering their inexpensive cost and ease of installation.